There is known a method for controlling DC transmission lines (cf. The Major Engineering Problems of High-Voltage D.C. Transmission, Developments in Science and Technology, Electrical and Power Engineering, Moscow, 1971), whereby a rectifier substation and an inverter substation are controlled so as to maintain a constant predetermined value of current through the transmission line. If the current exceeds the predetermined value, voltage at the sending end is reduced by increasing the lag angles of the bridges of the rectifier substation; if the current is lower than the predetermined value, voltage at the receiving end is reduced by increasing the lead angles of the bridges of the inverter substation. The method under review provides for protection from excessive current with substations operating at minimum permissible closing angles and thus ensures optimum operating conditions for the transmission line and branch systems. However, protection from excessive current requires the use of telecommunication for coordinated changes of the current settings of current regulators at both substations. On the other hand, a telecommunication channel necessitates considerable investment, keeping in mind great lengths of DC transmission lines and stringent reliability requirements.
There is known another method for controlling DC transmission lines (cf. accepted Japanese Application No. 35734/72, concerned with a system for controlling a high-voltage DC transmission line during evening hours). According to this method, one of the substations is controlled so as to maintain constant voltage, while the other is controlled so as to maintain constant current. There is no need to use telecommunication for effective protection from excessive current, but the end substations operate with closing angles of the converters that are much greater than the minimum possible angles. This is not the best method from the viewpoint of power transmission because it entails substantial power losses, calls for high-power compensation devices and reduces the utilization factor of the equipment.
The aforedescribed accepted Japanese application is further concerned with a DC transmission line comprising a rectifier substation, a system for phase control of the closing angle of the gates of the rectifier substation, a current transducer and a current regulator of said rectifier substation. The DC transmission line includes an inverter substation, a system for phase control of the closing angle of its gates, a voltage transducer and a rectified voltage regulator of the inverter substation.
The control means of the DC transmission line under review suffer from all the disadvantages inherent in the method they are called upon to carry out.